1. Field of the Invention
The present invention relates to a method of creating a mechanical force as a novel fluid drive source by utilizing the surface tension difference convection that is spontaneously generated between two fluids having an interface, such as Fluorinert and silicone oil, and to a technique for applying this force to a micromachine.
2. Description of the Related Art
Nowadays processing at the micro level is required in a wide range of fields such as the semiconductor field or medical fields and various types of so-called micromachines have been developed to achieve this. Various microstructures are created by employing micro-optical molding or CVD techniques making use of an ion beam or electron beam for forming machine structures. However, regarding the mechanism for driving such micromachines, their very minuteness results in various circumstances which normally do not create problems becoming barriers which make it difficult to put such machines into practice. For example, on the micro scale, frictional force becomes dominant, so idle rotation disengaging the microdrive sources that have been developed up to the present from excessive load is difficult to achieve, giving rise to the problem of deformation or destruction of the machine.
In recent years, the μTAS (Micro Total Analyses) technique, in which a chromatograph and other items are incorporated on a chip of dimensions of a few centimeters in order to analyze minute samples of for example blood has come into favor. In view of the advantages such as that diagnosis of for example diseases can be performed at home, that only a very small sample is required, and that the results are immediately available, a new technical field is being established. However, when for example a test reagent is added to this μTAS, the physical phenomenon arises that the Reynolds number becomes small when the representative length becomes small, on account of increased viscous effects on the micro scale; as a result, laminar flow takes place on the μTAS, so that the reagent fails to mix with the sample. On the ordinary scale, there are techniques for facilitating the mixing of two liquids, but, on the μTAS, due to the fineness of the scale, it is difficult to mix two liquids. There is therefore an urgent demand for development of mixing mechanisms capable of being employed in the micro world. (See Seungbae Hong, Luc G. Frechette and Vijay Modi “Numerical simulation of mixing in a micro channel with nonuniform zeta potential surface”: Proceedings of the Micro Total Analysis System 2002, Volume 1, p 94 (2002))
The present group of inventors carried out repeated studies based on the concept of utilizing Marangoni convection as a drive source for such micromachines. The Marangoni phenomenon refers to the phenomenon that the surface tension becomes nonuniform when a temperature gradient or concentration gradient is produced at an interface between gas and liquid, or to the phenomenon that flow is thereby created. For example, when a small piece of detergent or camphor is placed on a water surface, it dissolves in the water, changing the surface tension of the water and, as a result, the piece moves over the water surface. Most people will remember the sight of a small boat with a piece of camphor attached to it dashing about at a fete; this utilizes the Marangoni phenomenon. Also, when nonuniformity of temperature occurs at the surface of a liquid of high volatility, such as ether, flow similar to Bernard convection spontaneously occurs close to the surface, due to nonuniformity of surface tension; this is referred to as Marangoni convection.
Under microgravity and on the micro scale wettability and surface tension are dominant; the group of the present inventors, noting the similarity of these, studied handling of fluids by controlling wettability and/or surface tension. There are many examples of studies of surface tension difference convection because of the effect this has on crystal quality in single crystal growth of semiconductors. Also, under microgravity, natural convection disappears and, instead, surface tension difference convection is enhanced and has therefore been studied. Studies have also previously been made concerning the generation of Marangoni convection due to temperature differences, concentration differences and distribution of surface potential. (See Katsuhiko Fujinawa “Rayleigh motion and Marangoni motion” Kagaku Kogaku (Chemical Engineering) Vol. 49, No. 11, p 896 to p 901 (1985)) Although in previous studies, it was understood that Marangoni convection required a difference in temperature or a difference in concentration, with the combination of silicone oil and Fluorinert used in the present study (hereinbelow this will be referred to as a silicone oil/Fluorinert system), there is substantially no mixing of these two substances, so an interface is created and the liquid/liquid/gas Marangoni convection generated between the two liquids, namely silicone oil and Fluorinert on the one hand and the atmosphere (gaseous phase) on the other is generated solely where the two liquids make contact. This phenomenon overturns the conventional wisdom regarding Marangoni convection in that it does not require temperature difference or concentration difference and no reports at all of this phenomenon have so far appeared either in Japan or overseas. It is a major characteristic feature of the liquid/liquid/gas Marangoni convection that occurred in the present studies that it is generated solely by contact of two liquids. Although this phenomenon has not at present been fully analyzed, it is thought that the drive force is produced by evaporation of Fluorinert. Under conditions of exposure to the atmosphere, there is no possibility of saturated vapor pressure being reached if two-fluid contact of silicone oil and Fluorinert is performed and drive can therefore be achieved so long as Fluorinert is present; in this respect there is a considerable difference in comparison with other conventionally known types of Marangoni convection. It may be noted that it is a necessary condition in the case of a small boat that moves under the action of camphor that there should be a difference of concentration: when the concentration of camphor in the water reaches a prescribed value, the boat ceases to move.